Every day, our brains face the challenge of combining information across discrete experiences to answer novel questions. This central ability emerges early in childhood and retains a high degree of importance throughout the lifespan. Yet, the mechanisms and brain regions that support the integration of information across distinct episodes have only recently become the subject of empirical study-even in adult populations. How the neural processes underlying this faculty develop throughout childhood and adolescence remains virtually unstudied. Recent work suggests that in the mature brain, both the medial temporal lobes (MTL) and ventromedial prefrontal cortex (VMPFC) are important for building and using knowledge that spans experiences. The proposed research will use high-resolution structural and functional magnetic resonance imaging in children, adolescents, and young adults to investigate how the ability to link memories across time changes as the brain matures. The key hypothesis is that despite extant notions that the MTL-based episodic memory system is fully developed early in childhood, memory integration-which requires dynamic interactions between MTL and VMPFC-does not emerge until adulthood. This hypothesis is consistent with the protracted structural development of MTL, VMPFC, and the white matter pathways connecting these regions. Furthermore, we hypothesize that the development of memory integration is critical for other important cognitive behaviors such as inferential reasoning. Aim 1 will determine how existing knowledge influences the ability to encode new, related information and will characterize the neural processes that support memory integration and inferential reasoning at different ages. Aim 2 will test the hypothesis that offline mechanisms demonstrated to support consolidation of individual memories also support strengthening of integrated memories that span experiences. We predict that evidence for offline consolidation of integrated memories will be observed only in the mature brain. Aim 3 will employ structural MRI and high-resolution diffusion-weighted imaging to determine how VMPFC, MTL subregions, and their associated white matter pathways develop to support behavior, thus providing a wealth of neuroanatomical data to inform memory theories. We anticipate that our findings will advance understanding of both typical and atypical development. Reasoning ability is predictive of both math and reading success; testing the link between memory and reasoning may thus provide new insights into the neural predictors of real-world academic achievement. Moreover, a number of clinical conditions are associated with marked memory deficits and structural abnormalities in MTL and prefrontal areas (e.g., autism, schizophrenia, depression). Understanding how these regions develop to reach adult-like functionality is critical for creating effective treatment interventions for these mental health disorders throughout the lifespan.